1. Technical Field
The present invention relates to image processors for providing image data to be printed at a printing device. In particular, the present invention relates to an image processor adapted to generate image data to be output based on comparisons between pixels of input image data and thresholds associated with the pixels in a dither matrix.
2. Background Arts
There have been typical known methods of using thresholds in a dither matrix for binarizing tone data of original image data to generate data representing pseudo halftones.
For binarizing colored original image data, the dither matrix employed is a dot dispersing dither matrix in which locations of dots are changed for every color component.
When this technique is applied to superimpose a plurality of colors on a dot, granular feeling is increased, and clearness due to muddy secondary colors is decreased. Moreover, in order to make the periodicity inconspicuous, it is necessary to provide a very large dither matrix requiring a large storage area in a memory.
On the other hand, there is a dot dispersing dither matrix having thresholds spatially shifted for every color component. However, in techniques employing such a dot dispersing dither matrix for binarizing colored original image data, the offset amounts are fixed, thereby limiting suppression ranges of superimposition of dots. Moreover, superimposed dots increase the quantity of ink printed per one pixel, with increase in penetration of ink through a printing sheet.
In this way, conventional techniques are subject to increase regions having different colors superimposed on a dot. This superimposition fails to produce a favorable color and reduce granular feeling. Such an inconvenient issue is unable to remove while reducing the required memory area. Japanese Patent Application Laid-Open Publication No. 7-290737 discloses a technique to solve such an issue with a reduced memory area for storing a dither matrix.
According to this technique, a basic dither matrix is stored in a memory area. The dither matrix has thresholds for application to a prescribed color component. Any threshold once applied to the prescribed color component is changed before application to a subsequent color component. This change is made in consideration of a tone value for the prescribed color component and a given maximum. Such an operation is repeated so that thresholds in a dither matrix applied to the subsequent color component are different in value from thresholds in the dither matrix applied to the prescribed color component.
Thus, despite use of a single dither matrix, which is stored in a memory, the dither matrix can be applied to respective color components and minimize superimposition of colors. This enables to suppress unclearness (with color dulling and granular feeling) due to superimposed colors in color printing.